New Low-Temperature NMR Studies Establish the Presence of a Second Equatorial−Apical Isomer of [(R,S)-Binaphos](CO)₂RhH

The important enantioselective hydroformylation catalyst [( R, S )-Binaphos](CO) 2 RhH has been reexamained by low-temperature NMR spectroscopy. Both 1 H and 31 P NMR spectroscopy at −90 °C allow direct observation of a mixture of two apical−equatorial chelates. The major chelate, 1 eq,ap, was shown to have an equatorial phosphine and an apical phosphite. Its structure was unambiguously assigned using low-temperature 31 P NMR spectroscopy with selective decoupling of aromatic hydrogens but not of the rhodium hydride, which showed a 225 Hz trans phosphite to RhH coupling. The equilibrium constant for [ 1 eq,ap ]/[ 1 ap,eq ] was determined over a wide temperature range.